linux-ext4 - Re: ext3_valid_block_bitmap: Invalid block bitmap in 2.6.25rc in memory

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Message-id: <20080414234059.GM3106@webber.adilger.int>
Date:	Mon, 14 Apr 2008 17:40:59 -0600
From:	Andreas Dilger <adilger@....com>
To:	Mingming Cao <cmm@...ibm.com>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-ext4@...r.kernel.org
Subject: Re: ext3_valid_block_bitmap: Invalid block bitmap in 2.6.25rc in	memory

On Apr 14, 2008  07:50 -0700, Mingming Cao wrote:
> On Sat, 2008-04-12 at 22:57 +0200, Andi Kleen wrote:
> > FYI, a system here running various 2.6.25rc kernels (latest upto rc7-git6) 
> > with longer uptimes suddenly decided to fsck one of its file systems
> > due to an error after reboot.
> > 
> > The error causing this was:
> > 
> > kernel: EXT3-fs error (device dm-0): ext3_valid_block_bitmap: Invalid block bitmap - block_group = 285, block = 9338882
> > 
> > detected by the 2.6.25rc7-git6 kernel.
> > 
> > I don't see any ill effects from it and fsck didn't find anything wrong
> > so it must have been something spurious in memory only (or fsck
> > fails to check for this condition, but that is hard to imagine) 
> 
> The ext3_valid_block_bitmap() is to check whether the block or inode
> bitmap block is marked as "used" in the block group bitmap, to prevent
> allocating blocks from these system meta data blocks.

Right.

> The error messages seems indicating that one of the block group meta
> data is corrupted, but I don't why fsck doesn't catch this, Andreas?

It might have been corrupted on read (e.g. bad cable, or bad/wrong
data read from disk the first time).

The message itself isn't very useful though.  It should report what it
thinks is wrong with the bitmap (e.g. whether block/inode bitmaps are
unallocated, which/how many itable blocks are unallocated).

> Mingming
> > The system never showed anything like this on earlier kernel versions.

This is a new check, to catch allocation bitmap corruption before it
causes the corruption to spread into the rest of the filesystem by
double-allocating blocks, etc.  Having a checksum would also be good,
but even then memory corruption can lead to a valid checksum of bad
data in memory so a validity check is still useful for such important
and rarely-read data.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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